The invention relates to a glow plug. Glow pencils for such glow plugs can be produced by first creating a green body by extrusion that has a core made of ceramic material that is electrically conductive after sintering, an intermediate layer that surrounds the core and is made of ceramic material that is electrically insulating after sintering, and a layer that surrounds the intermediate layer and is made of ceramic material that is electrically conductive after sintering. Once the green body has been sintered, the outer layer forms the outer conductor of the glow pencil and the core forms the inner conductor of the glow pencil. In order to provide the glow pencil with a heating portion, the outer layer is removed from an end portion of the green body before sintering and this end portion is enclosed by a layer made of another ceramic material that is electrically conductive after sintering. This layer can be applied as slip and may be called a slip layer. By sintering the green body, an outer conductor layer having increased electrical resistance, that is to say, a heating layer, is formed from the slip layer.
Such glow pencils are sometimes referred to as outwardly heating glow pencils, since the heating resistor is provided as a portion of the outer conductor. This has advantages compared to inwardly heating glow pencils, in which a portion of the inner conductor is provided as a heating resistor. More specifically, in outwardly heating glow pencils the heat generated in the heating resistor can be emitted very quickly and efficiently to a fuel/air mixture in the combustion chamber of an engine.
The surface temperature of outwardly heating glow pencils responds very quickly to a change in the heating power, and therefore outwardly heating glow plugs can be controlled using modern glow plug control devices in accordance with the power stroke of an internal combustion engine. Modern glow plugs and glow plug control devices can thus assist fuel combustion efficiently and can adapt the heating process to individual requirements of a motor, for example, so as to increase the performance thereof or to reduce the exhaust emissions thereof.